Man, this is a tough issue. It covers a HUGE number of people in a HUGE number of different circumstances. Invariably some people (probably mostly kids) are going to suffer for stuff that is absolutely not their fault. Ultimately it would be nice if we could apply some common sense to individual cases, but I don't see any real way to do that. But ultimately the problem is terribly complex and difficult.
I can tell you that from the people around my part of the world, that getting citizenship from Mexico is difficult. There are a lot of people here that definitely contribute to society and deserve to be here. There are also a lot of ****ing shitbags that need to GTFO, but that is not exclusive to immigrants.
On the assimilation issue, there are people around here that suggest that assimilation won't be an issue in a generation, because as stated above, the children will do it. Again, that's a pretty wide generalization applied to a large diverse group. But assimilation doesn't always occur through time. My great great grandparents came over from Germany in the nineteen teens. Grandpa always remembered that they knew German fluently but absolutely refused to speak it because of the war. Sometimes a reason to disassociate is necessary to assimilate.